In recent years, forced or induced combustion furnace systems have become standard in residential use as a result of legislated minimum efficiency requirements. Minimum efficiency requirements, together with the desire to conserve energy, has led to the development of higher efficiency furnaces. It is generally known that in the operation of a gas fired furnace, combustion efficiency can be optimized by maintaining a specific ratio of fuel input flow rate and combustion air flow rate. Generally, the ideal ratio is offset somewhat for safety purposes by providing slightly more combustion air (conventionally referred to as "excess air") than that normally required for optimum combustion efficiency. Too much excess air, however, can result in furnace heat loss. It is therefore desirable to control excess air to minimize heat loss. It is known that the flow of combustion gases through the furnace's heat exchanger produces a pressure drop across the heat exchanger and that the pressure drop across the furnace's heat exchanger is proportional to total flow. Therefore, maintaining a desired flow, i.e., pressure drop, across the heat exchanger is critical to maintain a desired level of excess air for a given fuel flow rate.
Numerous factors, however, affect the critical nature of pressures and flows through a heat exchanger. Clearly, the basic design of a heat exchanger establishes its basic operating characteristics. A furnace's installation and setup, however, also have an impact on the pressure drop across the heat exchanger. For instance, factors such as the size and length of an exhaust pipe, as well as its configuration (i.e., number of elbows) can affect flow through the heat exchanger. Further, environmental conditions, such as altitude and temperature (which affect atmospheric pressure), even under pressure in a vent system, affect the flow and pressure through a heat exchanger. Still further, operating conditions such as dust build-up on an inducer fan, voltage variations on the power line or even bearing problems can affect the operation of the inducer blower and thus the pressure drop across a heat exchanger. Each of the foregoing create design and installation problems in maintaining a desired air flow through the heat exchanger.
Control systems have been suggested which would vary the speed of an inducer blower based upon sensed changes in the pressure drop across a heat exchanger. To date, however, such systems have not proved satisfactory in the marketplace based primarily upon the cost and reliability of sensors which can monitor pressure levels at desired locations in the heat exchanger. In this respect, the operative parts of a sensor are exposed to and must operate in an environment of corrosive combustion gases.
Another problem related to the use of pressure sensors in furnace applications is that the accuracy of such sensors is in many instances affected by the ambient "noise" or "vibration" typically associated with furnace operation. In this respect, pressure sensors typically include a movable diaphragm having sensing means attached thereto. Vibration noise created by the blower and inducer motor, or even by the rapid flexing of metal panels upon ignition of a burner, can produce movement of the diaphragm. (i.e., "flutter") which in turn affects the accuracy of the sensor signal.
The present invention overcomes these and other problems and provides a flow control system for regulating flow of a heat transfer fluid in a heat transfer system, such as a fuel combustion system, in response to sensed pressure differentials or flow at predetermined locations within such systems. In addition, the present invention provides a sensor which is less sensitive to vibration noise, yet is reliable, accurate and relatively inexpensive to manufacture.